A Pulse Forming Converter (“PFC”) is an electronic circuit that generates high power current pulses or voltage pulses that are delivered to an electrical load. Part of the goal of a PFC is to shape electrical pulses in terms of amplitude, pulse width, and duty cycle. PFC's are utilized to drive a variety of loads, including resistive loads, leading and lagging power factor loads and non-linear loads such as high power laser diodes. However, currently many types of PFC's operate with relatively low efficiency, and some even require a great deal of cooling support hardware.
Designers of PFC's esteem the following characteristics of PFC's, which heretofore have been elusive to achieve with today's circuits:                1. Generate current pulses (or voltage pulses) with precisely controlled amplitude and/or pulse width;        2. Programmable pulse amplitude;        3. Programmable pulse width;        4. Programmable duty cycle or repetition rate;        5. High efficiency;        6. Lightweight;        7. Fast pulse rise time and fall time;        8. Low current ripple;        
FIG. 1 shows a linear pulse generator 10 that is a foundation example of the elements in a PFC. It consists of an amplifier which is a control mechanism 11 having a current command input 11a, a current sense device 12, a power transistor 13 and a power source 14 shown as a battery. The amplifier uses feedback to compare the sensed current with a current command and adjusts the drive to the power transistor to obtain the desired pulse amplitude and pulse width at the load. Such loads 16 may vary, but are shown in the Figure as a series of laser diodes. For clarity of discussion, FIG. 1A shows a typical current pulse with portions defined that are important characteristics for a PFC.
One undesirable characteristic of linear pulse generators is high power dissipation. The power dissipated in the power transistor is equal to the product of the voltage across the transistor switch 13 times the load current 17. This high power dissipation limits the amount of power that can be obtained from this device. Cooling hardware that is heavy and occupies a large volume may even be needed to maintain an acceptable operating temperature in the power transistor 13.
FIG. 2 shows a good example of pulse forming network 20 utilizing a Buck switching converter. The Buck switching converter shown is used to regulate direct current (DC) in a load by regulating a DC current to a load 21 that is equal to a steady state commanded current. The load 21 can be a resistor, or a reactive load such as a resistor and capacitor in parallel. It can also be any of several electrical devices including DC motors and laser diodes. The power source 22 is shown as a battery 23 with series resistance Rs 24. The power can be from other sources including a DC generator or rectified utility power. Capacitor C1 26 reduces ripple on input voltage Vin 27.
Operation of the Buck switching converter is as follows. An oscillator 28 sends out pulses at a fixed frequency. The first pulse sets the output Q 29 of the flip/flop 31 high. This turns on transistor switch 32, which is shown as a bi-polar transistor, but may be other suitable transistors such as field effect transistors (“FETs”) or power MOSFETs. A voltage equal to (Vin−Vo) is applied across the inductor L1 33. The current in L1 increases at a rate defined by (Vin−Vo)/L1.
The current in the transistor switch is measured by current sensor 34. The sensed current is compared to current established by commanded current 36 at comparator 37. When the sensed current exceeds the commanded current, the output of the comparator 37 resets the output Q 29 of the flip/flop 31 to a low value that turns off the transistor switch 32. Diode D1 38 conducts and provides a path for current to continue to flow through the inductor to the load 21 after switch 32 has been turned off. With the transistor 32 off, the inductor current decreases at a rate of Vo/L1.
When the next pulse is sent by the oscillator 28, the transistor 32 is turned on and the process repeats. In this way, the Buck switching converter 20 can regulate peak current into a load. This control method is known as “pulse-width-modulation” because the “on” time of the transistor is modulated to control the output.
The Buck switching converter 20 can be used as a pulse generator by gating it on and off. An advantage of the Buck switching converter 20 over the linear pulse generator 10 is high efficiency. Because the transistor switch is either ON or OFF, it has low power dissipation. This reduces the cooling requirement.
A disadvantage of the Buck switching converter pulse generator 20 is slow rise time. The pulse rise time is inversely proportional to the inductor value. In other words, decreasing the value of L1 reduces rise time. The penalty for decreasing the value of L1 is increased load ripple current.
Therefore, what is needed is a pulse forming converter that has improved efficiency over existing designs while maintaining fast waveform rise times, low ripple current in the load, and low weight and size.